The invention relates to a two-wire switch with a power transistor, in particular, a VMOS transistor, which is arranged in series with an auxiliary impedance between two switch connection lines which are connectable to the poles of a dc voltage source for generation of a dc voltage pulsating between zero and a maximum value, with an, in particular, contactlessly, externally influenceable initiator for generation of blocking and driving signals for the transistor, and with an auxiliary voltage source for generation of an auxiliary voltage pulsating with the frequency of the pulsating dc voltage.
Such a two-wire switch is known from German Offenlegungsschrift No. 3,320,975. The known two-wire switch comprises as auxiliary impedance, in particular, a diode connected in series with the power transistor, with a capacitor serving as buffer capacitor to generate an input voltage for a dc-dc converter connected in parallel therewith.
An auxiliary voltage which may be used as supply voltage for the oscillator of the initiator, as supply voltage for an amplifier and, possibly, as control voltage for the power transistor, is gained from the voltage dropping over the diode, in the activated state of the power transistor, with the aid of the dc-dc converter by inductive or capacitive voltage multiplication.
In the known solution, the fact that generation of the auxiliary voltage involves an increased voltage drop over the switching path may prove to be a problem, particularly if the power transistor is a VMOS transistor requiring a high control voltage. In this case, allowance should be made for the fact that the voltage over the diode may, on account of the voltage multiplication in the converter, be smaller than the auxiliary and control voltages ultimately required, but, on the other hand, it must be high enough for not only a sufficiently high voltage, but altogether a sufficiently high energy to be available on the output side of the converter.
On the basis of this prior art, the object underlying the invention is to so improve a two-wire switch of the kind mentioned at the outset that the voltage drop in the load circuit of the switch is reducible to a minimum.